1. Field of the Invention
The present invention relates to a coordinate reading apparatus, i.e., a so-called digitizer, more particularly, a coordinate reading apparatus in which coordinates corresponding to a specified position on a tablet are read by detecting the phases of rotating magnetic fields induced above the tablet by means of a plurality of magnetic-field generating coils.
2. Description of the Prior Art
A coordinate reading apparatus is a device in which coordinates of a pen, i.e., a position detector located on a table, are read. Such a coordinate reading apparatus is useful as a graphic information processing system which is very important in many technical fields, for example, automatic design or automatic drawing.
In the coordinate reading apparatus of the prior art, three methods have been proposed and put into practical use. The first is a method based on an electrostatic coupling between the tablet and the position detector, the second is a method based on the electromagnetic induction between the tablet and the position detector, and the third is a method based on the ohmic resistance obtained from the tablet.
A coordinate reading apparatus constructed according to the second method, i.e., the electromagnetic induction method, and achieving the detection of position by utilizing the phases of rotating magnetic fields has been proposed in U.S. Ser. No. 238,961, filed Feb. 27, 1981, now U.S. Pat. No. 4,418,242. The coordinate reading apparatus of U.S. Pat. No. No. 4,418,242 is comprised of: a plurality of magnetic-field generating coils arranged in a tablet in parallel with each other; an alternating signal generator which supplies to the coils a plurality of driving signals, the phases of which are shifted one by one with respect to each other, so as to induce the rotating magnetic fields above the tablet, the phases of which differ from each other in accordance with the positions on the tablet; and a magnetic-field detector, contained in a pen, which is located above the tablet and detects the rotating magnetic fields. The rotating magnetic fields are formed so that the phases continuously change from 0.degree. to 360.degree. along positions from one side of the tablet to the other.
Thus, it is possible to determine the coordinates at which a pen is positioned on the tablet by determining the phase detected by the magnetic-field detector.
The coordinate reading apparatus of U.S. Pat. No. 4,418,242 will be explained in further detail hereinafter, however, that apparatus is not considered to be accurate enough. To be more specific, while the phases of the rotating magnetic fields should change linearly from one side of the tablet to the other, they actually change in an approximately S shaped pattern. The desired linearity has only been obtained at the center of the tablet.
Under such circumstances, it has been desired to realize a coordinate reading apparatus which can produce the desired linearity over the entire surface of the tablet.